1. Field of the Invention
The present invention relates generally to electro-mechanical energy converters.
2. Discussion of Background Information
One type of electro-mechanical energy converter, called an “electric motor,” converts electric energy into mechanical work. Another type of electro-mechanical energy converter, called an “electric generator,” converts mechanical work into electric energy. Both types of electro-mechanical energy converters come in a range of sizes and are often interchangeable in operation, which is to say that a motor can act as a generator and vice versa when the process is reversed. In all cases, mechanical work is required to drive an electric generator that can come from a variety of sources, amongst which is the work provided by ocean waves.
Motors and generators typically operate at high speed (1000 to 4000 rpm) and low torque because this combination reduces the overall cost to manufacture for a given power level. The relatively slow speed and large forces from ocean waves result in challenging requirements for electro-mechanical energy conversion. Direct mechanical coupling of these low speed (less than 5 rpm revolution per minute) and high torque (millions of Newton-meter) mechanical forces and converting it to electrical energy can be efficiently and cost effectively achieved with a large-diameter direct-drive generator. This direct coupling requires that certain electromagnetic and mechanical design challenges be addressed.
A common industry practice to address low-speed and high torque motor/generator requirements is to increase diameter. An increase in diameter improves both efficiency and reduces the unit material cost for the same torque delivered by the motor/generator. Torque is improved by a large diameter machine due to the increased machine radius and a longer lever arm acting on the same electromagnetic force.
In a conventional design, a generator/motor consists of two primary components: a fixed element, called a “stator,” against which a rotational element, called a “rotor” electromagnetically reacts. The stator and rotor are separated by a small radial clearance (air gap) that provides mechanical clearance between the moving parts. Through numerous machine design types, understood by those skilled in the art, magnetic flux is directed through the air gap between stator and rotor and through one or more sets of metallic coils. The relative rotation between stator and rotor causes a time rate of change of the magnetic flux through the metallic coils and generates voltage directly proportional to that rate of change. The time rate of change of magnetic flux can be increased either by faster rotation at the air gap and/or by higher flux density. For a given rotational speed, the velocity is proportional to the radius, which means that the larger the diameter of the generator/motor, the faster the relative motion between the rotor and stator at the air gap. It can be shown that when all other machine parameters are assumed constant a higher speed translates into higher flux velocity and improved generator efficiency.
As the diameter of an electro-mechanical energy converter increases, the ability to manufacture these parts precisely (i.e., with smaller or “tighter” tolerances) and therefore the ability to maintain a small air gap becomes increasingly difficult and more expensive. Tolerances of approximately 5 to 10 millimeters (mm) have been achieved with existing large diameter direct-drive generators/motors. Large air gaps, such as 5 to 10 mm, decrease the efficiency (and/or increase the cost) of a motor/generator.
A need therefore exists for an increased motor/generator diameter and a need for a design that allows this large diameter with a reduced air gap (e.g., 0.5 mm to 3 mm) between stator and rotor in an electro-mechanical energy converter.